1. New Technology Research Centre, University of West Bohemia, Univerzitni 8, 306 14, Pilsen, Czech Republic.
2. Department of Physics and Astronomy, Ames Laboratory, Microelectronics Research Center and Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011, USA.
3. Department of Physics, University of Science and Technology of China, Hefei, China.